A polymeric molding material is subjected to molding by itself or after mixing with another additive. For example, in the case of a vinyl chloride resin, it is well known that the vinyl chloride resin is subjected to molding after powdery mixing with a polymeric modifier, such as an impact modifier comprising a rubber-based graft copolymer, a fusion promoter-type processing aid comprising, e.g., an acrylic resin having a relatively high glass transition temperature (Tg), or a lubricant-type processing aid comprising, e.g., an acrylic resin having a relatively low Tg. Such a polymeric modifier is recovered in a powder form from a latex of the polymer, and for its use as a molding material or for the powdery mixing thereof prior thereto, should desirably have good powder properties as represented by a narrow particle size distribution, and excellent flowability and anti-blocking property. However, such desires have not been necessarily met so far.
For example, regarding the lubricant-type polymeric processing aid, multilayer polymer structures comprising a low-Tg polymer and a high-Tg polymer and containing a relatively large proportion of the low-Tg polymer, have been proposed (JP-B 50-37699, JP-A 49-120945, JP-A 50-9653), but the powder properties thereof have not been satisfactorily improved.
More specifically, as for methods of obtaining powdery or particulate products from polymer latexes, there have been used a method of mixing a polymer latex with an electrolyte aqueous solution under stirring to coagulate the resin content, a method of spraying a polymer latex into a hot gas stream to dry the polymer latex, etc.
However, the powdery products obtained through the above methods contain a large amount of fine powder fraction, so that they are accompanied with many problems in handling, such as poor filterability or dewaterbility, and dusting after drying (i.e., scattering of dust during operations, such as transportation, metering and loading of powdery products) resulting in poor operation environments.
In contrast thereto, several methods including two-step coagulation of polymer latexes for recovering powdery or particulate products having good powder properties from such polymer latexes, have been proposed (JP-A 59-91103, JP-A 60-217224, JP-A 6-24009, etc.). These methods are understood as having succeeded to some extent in recovery of powdery polymers having improved powder properties from crosslinked rubber-based latexes, but it is difficult to regard that these methods have succeeded as methods for recovering linear polymer powders. In contrast thereto, our research and development group has succeeded in recovering a powdery polymer having good powder properties from a latex of an acrylic linear polymer having a relatively high glass transition temperature (JP-A 10-017626) but has not succeeded in recovering a powdery polymer having good powder properties from a latex of a linear polymer containing 30 wt. % or more of polymer component having a glass transition temperature below 40° C.